This invention relates to brush assemblies for dynamoelectric machines and more particularly to such assemblies for use in homopolar generators.
Homopolar generators have been successfully designed for providing short duration pulses having a peak current in excess of 1,000,000 amperes DC. Such generators generally include a cylindrical rotor of either a drum or disk configuration, mounted on a frame and rotated about a central axis. A field coil encircling the rotor and connected to an external current supply provides and applied magnetic field excitation passing through the rotor. The applied field excitation is usually confined and direction by a ferromagnetic stator structure surrounding the field coil and all, or a portion of, the rotor. When the rotor is spinning, free electrons within the rotor experience an electromotive force resulting from their interaction with the applied magnetic field excitation. Brushes, positioned adjacent to a current collection zone on the rotor, are then lowered onto the spinning rotor to allow an electrical current to flow under the influence of the electromotive force through return conductors to an external circuit, and then back onto the rotor through additional brushes at a different location. During the discharge, the interaction of the discharged current in the applied field creates a force which decelerates the rotor. It has been found, that extremely high current pulses may be obtained by using a relatively low power conventional prime mover or a conventional low voltage, low amperage power source to store initial energy in the rotor by gradually motoring the rotor up to the desired rotational speed.
Drum type homopolar dynamoelectric machines include a stationary excitation system and a rotating drum composed of a combination of ferromagnetic and highly conductive materials such that a direct current output voltage is produced along the axial length of the drum. These machines incorporate a set of current collecting brushes at axially displaced locations along the rotor's surface, which carry full load current. Homopolar dynamoelectric machines may operate as either a motor or a generator and are particularly suited to transfer energy in short, high current pulses to a storage inductor and a final load consisting of a resistive-inductive system. The rotor of drum type homopolar machines may include a cylindrical shell of a highly conductive, nonferromagnetic material which generates and supports the full load current. This member is bonded or shrunk onto a ferrogagnetic inner cylindrical core which serves as the main rotor body and is directly attached to a drive or input shaft. Both components of the rotor are, preferably, homogeneous materials without segmentation or any combination of axial or circumferential grooves. Since modern current collectors may operate at a current density of between 10 and 15 kiloamps per square inch, it is imperative that the rotor surface near the two axial ends be smooth since this zone is used exclusively for current collection with, for example, solid metal graphite or fiber brushes. The machine's internal electromotive force is confined to an axial zone along the center of the rotor between the two outer current collection zones.
The rotor surface speed of a drum rotor in a high performance homopolar generator may exceed 100 meters per second and the generated current may exceed 1,000,000 amperes. Under these conditions, it is important that the current collecting brush members make intimate sliding contact with the rotor's surface. Prior art current collecting brush assemblies have included a plurality of contact members each having an individual pin and spring assembly which provided a spring force in a generally radial direction with respect to the rotor such that when the contact elements were lowered onto the rotor, the individual spring and pin assemblies forced the contact elements into sliding contact with the rotor. That brush configuration was relatively costly to manufacture, machine and assemble. In addition, the brushes were weakened by the required machining and therefore more susceptible to failure.